Treatment of liquiefied petroleum gas



United States Patent Ofifice 2,693,443 Patented Nov. 2, 1954 TREATMENT OF LIQUEFIED PETROLEUM GAS Mathis T. Waddell, Baytown, and Jean H. Karchmer, Houston, Tex., assignors, by mesne assignments, to Standard Oil Development Company, Elizabeth, N. J., a corporation of Delaware No Drawing. Application May 19, 1951, Serial No. 227,274

10 Claims. (Cl. 196-32) The present invention is directed to a method for removing sulfur from liquefied hydrocarbon gas. More particularly, the invention is directed to a method for removing free sulfur from liquefied petroleum gas, such as propane and butane, and mixtures thereof.

The invention may be described briefly as involving the treating or contacting of liquefied petroleum gas containing free sulfur in excess of about one part per million with a solution of an alkali metal hydroxide to which has been added an amount of an alkyl mercaptan having no more than 2 carbon atoms in the molecule in the range from 0.8 to 4 percent by weight based on the alkali metal hydroxide. The solution of alkali metal hydroxide is then separated from the liquefied gas which has been substantially freed of elementary sulfur and mercaptans. The amount of free sulfur remaining in the liquefied gas is less than about 1 p. p. m. which meets specifications for this product.

The liquefied hydrocarbon is a petroleum gas, such as propane or butane, or mixtures thereof, which may be liquefied and employed as a fuel. Such petroleum gases may have free sulfur contents in excess of 1 p. p. m., for example, 2 to 5 p. p. m. or more. The petroleum gas may be obtained as such from earth formations or it may be distilled from crude petroleum or from cracked products, and the like.

The temperature at which the treatment, in accordance with the present invention, takes place ordinarily will include a temperature in the range from about 40 F. to about 120 F. Suitably atmospheric temperature of about 70 to 90 F. may be employed.

The treatment may be conducted in a liquid phase. It will be necessary to superimpose upon the treating equipment containing the liquefied petroleum gas and the treating reagent sufiicient pressure to maintain a liquid phase. The pressure employed will depend, of course, on the treating temperature. The contacting may be conducted in contacting towers equipped with internal contacting equipment, in incorporators, mixers, trnhixing pumps, and other mixing devices well known to e art.

The concentration of alkali metal hydroxide in the solution should be sufficient to provide an alkaline solution having a pH in excess of 8.5. An aqueous solution of sodium hydroxide containing percent by weight of sodium hydroxide has been found to give good results. The alkali metal hydroxide may be either sodium, potassium, or lithium hydroxide, and preferably sodium hydroxide, by virtue of its cheapness and availability, although the other alkali metal hydroxides may be used in lieu thereof. The amount of the alkali metal hydroxide solution employed to contact the hydrocarbon containing free sulfur may range from about 5 to 30 percent by volume of the hydrocarbon being treated.

Suitable alkyl mercaptans for use in our invention are methyl and ethyl mercaptan. The higher alkyl mercaptans are unsuitable as they are extracted from the alkali metal hydroxide solution by the liquefied hydrocarbon gas.

The amount of the alkyl mercaptan, such as methyl or ethyl mercaptan, employed in the aqueous solution of alkali metal hydroxide to remove free sulfur from liquefied hydrocarbon gases containing it may range from about 0.8 to about 4 percent by weight based on the alkali metal hydroxide solution. Excellent results may be obtained in treating free sulfur containing liquefied hydrocarbon gases with a sodium hydroxide solution containing 10 percent by weight of sodium hydroxide to which has been added 1.5 percent by weight of methyl mercaptan.

The alkyl mercaptan may be added directly to the alkali metal hydroxide solution, such as sodium hydroxide solution, or may be extracted from the liquefied petroleum gas itself provided the liquefied petroleum gas contains methyl and/or ethyl mercaptan. When the alkyl mercaptan is extracted from the liquefied petroleum gas containing mercaptans and free sulfur, it is desirable to maintain in the sodium hydroxide or caustic solution on a regenerated caustic basis a sufiicient amount of mercaptans to provide therein a copper number in the range from 600 to 2000 copper number. This is equivalent to an amount of sodium alkyl mercaptides when the caustic solution is 10 percent by weight aqueous sodium hydroxide in the range from 1.17% to 4.50% by weight of the solution. On a basis of alkyl mercaptans this corresponds to the addition of the caustic solution of an amount in the range from 0.80% to 3.37% of alkyl mercaptans based on the caustic.

The copper number to which reference has been made is determined in accordance with the method described in U. 0. P. Laboratory Test Methods for Petroleum and' Its Products, Universal Oil Products Company, Chicago, 1940, page H-61. This test is a measure of the mercaptan sulfur content, usually of petroleum distillates, and is equivalent to milligrams of mercaptan sulfur per cc. of the sample tested.

In order to illustrate the present invention further, liquefied petroleum gas containing a substantial amount of propane was contacted in a commercial installation with a sodium hydroxide solution which contained sodium alkyl mercaptide, sodium hydroxide and sodium sulfide as a result of previous contact with liquefied petroleum gas containing mercaptans which were predominantly methyl mercaptan and probably some ethyl mercaptan. This stock contained in excess of two parts per million of free sulfur. By virtue of contacting the liquefied petroleum gas containing in excess of two parts per million of free sulfur with a sodium hydroxide solution which contained sodium alkyl mercaptides resulting from the extraction of alkyl mercaptan from the liquefied petroleum gas, it was possible to obtain a product having a free sulfur content less than about 1 part per million and containing little or no mercaptans. The caustic solution employed in this run was found to have added thereto methyl mercaptan, by extraction from the liquefied petroleum gas, analyzing in excess of 99 percent and a trace of ethyl mercaptan.

A number of tests were made on the product stream of propane leaving the unit in which the liquefied propane was treated as described above. Prior to treatment with the sodium hydroxide containing alkyl mercaptan the propane had a copper strip corrosion number which ranged from about 6 to 10, whereas after treatment in accordance with the present invention, the corrosion number average about 3.

In a further illustration of the present invention, a quantity of propane having a corrosion number of 8, which is unsatisfactory, was treated in a continuous process in a commercial unit with about 30 volume percent of a 10% sodium hydroxide solution to which approximately 0.8 volume percent, based on the caustic, of technical grade ethyl mercaptan had been added. The treat was carried out by passing the propane through the treater at a rate of 2000 barrels per day and recirculating the caustic through the treater at a rate of 600 barrels per day. During a prolonged period when the copper number of the caustic solution on a regenerated basis ranged from 400 to 320, corresponding to a range from 0.92 to 0.74 volume percent of ethyl mercaptan, the copper strip corrosion number of the propane so produced ranged from 1 to 3, which is well within the specifications.

The copper strip corrosion number of hydrocarbons which contain free sulfur is rated from 1 to 10 depending on the degree and type of discoloration (corrosion) obtained when a clean polished strip of copper is placed in a sample maintained at 122 F. for three hours. Liquefiedpetrol'eum ga's samples having a copper strip corrosion number of one to six are considered satisfactory.

From the foregoing runs, it will be seen that the present invention allows the obtaining of a product having a satisfactory corrosion number. This product contained less than 1 part per million of elementary sulfur.

The nature and objects of the present invention having been completely described and illustrated, what we wish to claim as new and useful and to secure by :Letters Patent is:

l. A method for treating a hydrocarbon fraction which comprises contacting a liquefied petroleum gas which contains more than 1 p. ,p. m. of free sulfur with a dilute aqueous solution of an alkali metal hydroxide which contains a small but-effective amount of an alkali salt :of an alkyl mercpta-n having no more than 2 carbon atoms in the molecule.

2. A method in accordance with claim 1 in which the mercaptan is ethyl mercaptan.

3. A method in accordance with claim 1 in which the mercaptan is methyl .mercaptan.

4. A method for removing free :sulfur from a liquefied petroleum gas containing more than 1 p. p. m. of free sulfur which comprises contacting said liquefied gas with a dilute aqueous solution of an alkali metal hydroxide which contains a small but effective amount of an alkali metal salt of an alkyl mercaptan having no more than two carbon atoms in the molecule in the range between 0.8 and 4% by weight calculated as the alkyl mercaptan.

5. A method in accordance with claim 4 in which the mercaptan is ethyl mercaptan.

6. A method in accordance with claim 4 in which the mercaptan is methyl mercaptan.

7-. A method in accordance with claim 4 in which the liquefied petroleum gas is propane.

8. A method for removing free sulfur from a liquefied petroleum gas containing more than 1 p. p. m. of free sulfur which comprises contacting said gas with an aqueous solution of 10% by weight of sodium hydroxide which contains sodium methyl mercaptide in an amount in the range from 0.8 to 4 percent by weight calculated as methyl mercaptan based on the sodium hydroxide, separating aqueous solution .from the liquefied gas, and recovering a liquefied gas containing less than 1 p. p. m. of free sulfur.

9. A method in accordance with claim 8 in which the liquefied petroleum gas is propane.

10. A method for removing free sulfur from a liquetied petroleum gas containing hydrocarbons having 3 to 4 carbon atoms in the molecule and containing free sulfur in an amount in .the range between 1 and 5 parts per million which comprises contacting said gas with an aqueous solution of 10% 'by Weight sodium hydroxide in an amount in the range :from about 5% to 30% by volume based on the liquefied petroleum gas which cont-ains sodium methyl mercaptide, calculated as methyl mercaptan, in an amount in the -range.from-0.8% to 4% by weight based on the sodium hydroxide, separating said aqueous solution from the contacted liquefied gas and recovering contacted liquefied gas containing less than 1 part per million of .free sulfur. 

